Electrical circuit control device



Oct. 26,1943. H SCHLEGEL 2,332,536

ELECTRICAL CIRCUIT CONTROL DEVICE Filed April 25, 1941 71 1 WW-W W ATTVS.

, would thereby be less pronounce Patented Oct. 26, 1943 ELECTRICAL CIRCUIT CONTROL DEVICE Karl Hermann Franz .Schlegel, Lyngby, ,Denmark; vested in the Alien Property Custodian Application April 23, 1941, Serial No. 390,013 In Denmark May9, 1940 11 Claims.

This'invention relates to electrical circuit'control devices and more particularly to control cirelectr'onic amplifiers in sound translating systems. An object of the invention is to provide an improved method of controlling the volume range in electro-acoustical translators.

Another object is to provide means for demodulating a control voltage used for automatic vol-v ume control so as to prevent sound frequencies from reaching the gain control member of the sound translating system.

Various other objects and advantages will become apparent ,to those skilled in the art as the nature of the invention is more fully disclosed.

In automatic volume control circuits in which the gain of an electronic amplifier is controlled by means of a control voltage derived from the input or output circuits of th sound translating channel, it is desirable that the control voltage should vary in accordance with the sound energy, and not, as is generally the case in the existing control circuits for automatic volume control, according to the peak amplitudes which occur in the signals.

The fact that in the volume control devices so far applied to automatic volume control the control voltage produced by rectification of the sound responsive electric oscillations varies as the envelope of the rectified amplitudes involves the drawback that occasional peak amplitudes occurring in a sound selection will have a Predominant effect on the control voltage. Thus, for instance, in-the case of. volume compression during the recording of sounds the peak ampli.- tudes will cause a sudden decrease of the gain of the amplifier, whereby the values of the subsequent amplitudes of the sound selection are reduced below the correct level, and the dynamic quality of the programme is thereby minimized. A substantial improvement in the dynamic quality would be achieved, if the control voltage could be caused to vary according to the sound ener y instead of following the envelope of the-rectified currents, as the effect of the peak amplitudes In electrical filter circuits comprising a rectifier, a parallel condenser, a series resistance and a load resistance, the control voltage across the condenser may be caused to vary proportionally with the mean value of the rectified amplitudes, provided that the charge time which is essencults for automatically controlling the gain of resistance are substantially of th same order. If the rectifier has a square law characteristic or if the input signal has beforehand undergone a deformation according to a logarithmic law, the D. C. control voltage derived from a circuit of this kind will vary according to the mean value of the squares of the amplitudes, i. e., in accordance with the energy of the sound responsive electric oscillations or, in other words, according to the sound energy.

However, the energy responsive control voltage derivedfrom such a circuit will contain audiofrequency fluctuations, and itis therefore necessary to provide means for demodulating the control voltage before it can be applied to the gain controlmember.

A feature of the invention consists in the use i of separate circuits for producing an energy responsive control voltage and for demodulating the said control voltage. y

In one embodiment of the invention the control voltage is derived from the first mentioned control circuit, wherein the voltage across. the condenser is caused to vary in accordance with the sound energy, and the demodulation is effect ed by suitable choice of the discharge time constant of a parallel condenser included in the tially determined by the series resistance and the discharge time constant determined by the load second circuit.

In another embodiment of th invention the demodulation of the control voltage is effected by providing means in the second circuit for maintaining the control voltage derived from the first mentioned circuit at a constant level during a predetermined time interval after the cessation of the sound amplitudes.

For a better understandingof the invention reference is made to the accompanying drawing in which Fig. 1 illustrates the curves of the signal current and the variations of a control current obtained by rectification of said signal current,

Fig. 2 illustrates a circuit for producing a con-' trol voltage proportional to the volume and free from audible modulation,

Fi 3 is a modified circuit of the same kind,

Fig. 4.illustrates a voltage-time curve of the condensers shown in the circuits in Figs. 2 and 3,

Fig. 5 is a diagram of the discharge time for two different values of the control voltage ac cording to the circuit shown in Fig. 3, and

. Fig. 6 is a modification of the circuit shown 7 in Fig. 3. K

Referring to the accompanying drawing, in Fig. 1, curve I shows the variation'of a signal voltage, 1. e., sound responsive electric oscillations during a time interval, whereas curve 2 shows the resulting variation after rectification and passage through a filter condenser. Provided that the frequency represented by curve I is the normally lowest frequency, for example 30 cycles, it will be seen that if the discharge time of the filter condenser is a relatively short one, for example of the same order as the charging time, the voltage across the condenser will start to fall immediately after the passing of one rectified current impulse. Curve 2 will thus follow the shape of curve I and will, therefor, have a definite modulation.

Fig. 2 shows rangement according to the invention in which the energy responsive control voltage is produced by feeding a portion of the sound responsive electric oscillations to a circuit comprising input terminals :11 and (:2, the rectifier E1, 9. series resistance R1, a parallel condenser C1 and a load resistance R2. The characteristic of rectifier E1 and the value of the resistances R1 and R2 which determine the charge and discharge times of condenser C1 respectively are matched in such away that the voltage variations across the condenser C1 are proportional to the volume of the signals, or, in other words, vary in accordance with the sound energy. Connected in series with the first mentioned circuit is the second circuit comprising a blocking rectifier E2, a parallel condenser C2 and a load resistance comprising the space discharge path of an electron discharge tube, preferably a pentode P. minals of the second circuit bl and in are connected with the anode and cathode respectively of the said pentode. The demodulation of the energy responsive control voltage produced across the condenser C1 is effected by means of the last mentioned filter circuit including the condenser C2, the discharge time constant of which may be chosen sufficiently long as to remove audible modulation from the control voltage. By loading the condenser C2 with the circuit of the pentode P the result is achieved that the control voltage across the condenser C2 decreases proportionally with the time after cessation of the sound responsive oscillations. It will be realised that instead of using the pentode P any equivalent load resistance such as a resistance in conjunction with a self-inductance may be applied. The embodiment shown in Fig. 2 may be used in such cases where conditions permit of a long discharge time oi the control voltage, as for instance when the reverberation of a room has to be corrected.

In many cases, however, for instance in electro-acoustical sound transmission using logarithmic compression, such a long discharge time as that of a circuit as shown in Fig. 2 will result in distortion. In such cases therefor, it is necessary to take special precautions to render the control voltage free from modulation. A modified circuit adapted to this purpose is shown in Fig. 3 in which the arrangement of the condenser discharge circuit supplying the control voltage is such that the discharge of the condenser is delayed during a suitable time interval so as to maintain the control voltage at a constant value during the said time interval, whereupon the condenser is rapidly discharged.

The operation of this circuit arrangement is as follows:

A portion of the sound responsive electric oscillations is impressed on the input terminals (11 and a: of the primary winding PW in a transformer T, the secondary winding SW of which is conan embodiment of a circuit ar- The output ternected with the anodes of two iodide rectifiers En connected in push-pull. The cathodes of the said rectifiers E are connected through a series resistance R1 with one terminal of a filter condenser C1, the charge time of which is essentially determined by the value of resistance R1, whereas the discharge time of the said condenser is determined by the value of the load resistance R2. The other terminal of condenser Cl is connected with the midpoint tap Q of the secondary winding SW. The values of the resistances R1 and R2 are so chosen that the charge time and discharge time of condenser C1 are approximately of the same order, for example 3 milliseconds and milliseconds. Assuming that the rectifiers E0 have a square law characteristic or that the sound responsive electric oscillations have undergone a logarithmic deformation, the voltage across the condenser C1 will consequently vary according to the sound energy. However, this voltage will be heavily modulated having superimposed on the D. C. component an A. C. component as shown in curve 2 of Fig. 1. The amplitudes 'of this A. C. component-an indication of the amount of modulation-depend partly on the frequency of the original sound responsive oscillations and'partly on the discharge time of the condenser C1, the modulation increasing the lower the frequency and the shorter the discharge time.

Parallel with the condenser C1 there is a blocking condenser C2 which is being charged at the same time through a diode rectifier E1. Now this blocking condenser C2, the function of which will be described in the following, is discharged considerably slower than condenser C1, namely, with a discharge time of the order of milliseconds which is caused by the load resistance of the blocking condenser. As in the embodiment shown in Fig. 2, the load resistance of condenser C2 is an electron discharge tube, preferably a pentode P, providing for a constant discharge current, whereby the voltage of the blocking condenser decreases proportionally with the time.

Referring again to Fig. 3 it will be seen that parallel with a part of the load resistance R2 is a third condenser Ca which is being charged through rectifier E2 with a voltage slightly lower than that across condensers C1 and C2, and which is connected to the anode of the pentode P through a blocking rectifier E3.

As long asthe voltage across the blocking condenser C2 is higher than the voltage across condenser C3, the latter will'have an infinite discharge time owing to the blocking rectifier E2 which will prevent discharge through the resistance R2. The voltage across the condenser C3 will therefor remain constant during a certain time interval, until the voltage across the blocking condenser C2 has decreased by discharge through the pentode P below the value of the voltage across condenser C3. At that moment. however, the last mentioned condenser will be discharged through the blocking rectifier E; and the pentode P.

The voltage variations across condensers C1. C2 and C: is illustrated in Fig. 4, wherein the corresponding voltages are indicated at e1, e2 and ex. The starting time of all the three voltage, is indicated by the time interval to-t1, the voltage e1 (varying as already mentioned in accordance with the sound energy) is heavily modulated owing to the short discharge time constant of condenser C1 and therefore cannot be used directly as a control voltage. The voltage (22 across the control voltages.

blocking condenser 02 of Fig. 3 follows mainly the 1 variations of voltage e1. Since, however, the condenser G2 has a larger discharge time constant the audible modulation will be much less. When conditions are arranged so as to provide for a long discharge time, as represented in Fig. 4 by the straight line u-tq which represents the discharge time of condensers C2 and C3, it may be possible to obtain a sufficiently low degree of modulation of the voltage e2 so as to permit this voltage to be used directly as a control voltage, as assumed in the embodiment shown in Fig. 2.

' The curve ea in Fig. 4 represents the voltage variations across the condenser C3 in Fig. 3. Owing to the function of the blocking condenser C2 in conjunction with the blocking rectifier E3 the control voltage 63 will remain constant withinv certain time intervals t1-tz and ta-ts and willconsequently be free from audible modulation. The discharge time-interval of condenser C1 is represented by t4ic whereas t4-t7 represents the discharge time of condenser C2. As is shown from the above, the value of the control voltage is given by the energy as distinct from the peak value of the sound responsive electric oscillations passing through the rectifiers E to condenser C1. The relation between the values of the control condenser C3 is in this case represented by the dotted line e;- "1 in Fig. 5. It will be seen that the constant voltage interval e'aa: corresponding to voltage e: is now equal to the constant voltage time interval t "v corresponding to volt-' 1 age e"a.

voltage e3, Fig.4, and the time interval during which the control voltage remains constant after cessation of the sound is illustrated in Fig. 5.

voltage across the responding interval t"v('- t4t"s) so that the stant voltage. interval Owing to the fact, that, in a musical programme the low notes which are of particular bigger the control voltage es the longer the con The modified circuit arrangement shown in Fig. 6 is similar to that of Fig. 3, there has been added, however, a rectifier circuit comprising an auxiliary winding SW2 on transformer T, a rec-' tifier E4, a parallel condenser C4 andan adjustable load resistance R3 the movable contact of which is connected to" the control grid G of the pentode P. A battery B supplying a negative bias e to one terminal of the said load resistance or potentiometer R3 is inserted in series with one of the output terminals b: of the control circuit. The operation of this circuit arrangement is as follows:

.Souhd responsive electric oscillations induced in secondary SW3 are rectified by rectifier E4 and the rectified pulses charge condenser C4. The voltage-across this condenser increases proportionally with the amplitudes of the input oscillations and opposes the negative bias on the control grid G so that the discharge current through the pentode P will increase proportionally with the increase of the input amplitudes. The rate of increase may be' arbitrarily adjusted by the setting of the slidable contact of resistance R3.

I claim: j

1. In a sound translating and amplifying system, a control device for variably controlling the gain of amplification comprising a rectifier and a primary filter circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses, a secondary filter circuit coupled to the said primary filter circuit, and blocking means inserted between the primary and secondary circuit for rendering the releasing time of the saidsecondary circuit independent of the time constants of the said primary circuit.

2. In a sound translating and amplifying system, a control device for variably controlling the importance with regard to the quality of the sounds generally occur with a large acoustical energy as compared with the high notes, the low notes will, in general, give rise to relatively large voltage time interval will normally be larger atlower than at higher frequencies of sponsive electric oscillations.

Since, on the other hand, the audible modulation is more pronounced in the control voltage derived from low notes the increased constant voltage time interval will normally provide for a satisfactory demodulation. I

In special cases,'however, such as in broadcasting of grammophone recording, the-low frequencies are often suppressed such that the lower notes do not occur with a relatively large acoustical energy. In such cases there is no reason for having different constant voltage time intervals at difierent control voltages, and it may, therefore, be desirable to provide means for keeping the constant voltage time interval independent of the value of the control voltage.

. In a further embodiment of the invention which ,is illustrated in Fig. 6 this result is achieved by providing means for increasing the discharge time of the control voltage condenser .C3 so that the discharge current increases with Consequently, the constant control voltage proportional to the mean value the sound regain of amplification comprising a rectifier and a primary filter circuit adapted to produce a of the rectified pulses, a. secondary filter circuit coupled to the said primary filter circuit and adapted to remove audible modulation from the said control voltage, and a blocking circuit for preventing the release of the said secondary circuit during a predetermined time interval so as to maintain the said control-voltage at a constant value during the said time interval.

3. In a sound translating and amplifying system, a control device for variably controlling the gain of amplification comprising a rectifier and a primary filter. circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses, a secondary filter circuit coupled to the said primary filter circuit and adapted to remove audible modulation from the said controlvolta'ge, a. blocking circuit for preventing the release of the said secondary circuit during a. predetermined time interval so as to maintain the said control voltage at a constant value during the said time interval and unidirectional means inserted between the said primary circuit and the said secondary and blocking circuits for rendering the discharge of the last mentioned circuits independent of the first mentioned circuit.

increasing amplitudes. 'I' 'he discharge of the.

4. In a sound translating and amplifying sys-' tem, a control device for variably controlling the gain of amplification comprising a rectifier and a filter circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses, a second filter circuit and a blocking circuitmeans for impressing a fraction of the said control voltage on the said second filter circuit, means for impressing a greater fraction of the said control voltage on the said blocking circuit, and uni-directional means inserted between the blocking circuit and the second filter circuit so as to prevent the release of the second filter circuit during a predetermined interval.

5. In a sound translating and amplifying system, a control device for variably controlling the gain of amplification comprising a rectifier and a primary filter circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses, a secondary to the said primary filter circuit and adapted to remove audible modulation from the said control voltage, and a blocking circuit for delaying the release of the said secondary circuit, said secondary circuit and said blocking circuit having separate condensers and a common discharge path, means for charging the condenser of the blocking circuit to a higher potential than the condenser of the secondary circuit, and uni-directional means inserted between the last mentioned condenser and the said discharge path so as to prevent the release of the secondary circuit durv ing a predetermined interval.

6. In a sound translating and amplifying system, a control device for variably controlling the gain of amplification comprising a rectifier and a primary filter circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses and including a condenser and a parallel resistance, a secondary filter circuit coupled to a part of said resistance and a blocking circuit coupled to a greater part of said resistance, said secondary circuit, and said blocking circuit having separate condensers and a common discharge path, and uni-directional means inserted between the condenser of the secondary circuit and the said discharge path so as to delay the release of the secondary circuit during a predetermined interval.

7. In a sound translating and amplifying sys- V tem, a control device for variably controlling the gain of amplification comprising an input transformer, a rectifier and a primary filter circuit adapted to produce a control voltage proportional to the mean value of the rectified pulses and including a condenser and a parallel resistance, a secondary filter circuit coupled to a part of said resistance and a blocking circuit coupled to a greater part of said resistance, said secondary circuit and said blocking circuithaving separate condensers and a common discharge path, a pentode having its anode-cathode resistance inserted in the said discharge path and uni-directional means inserted between the condenser of the said secondary circuit and the anode circuit of the said pentode so as to prevent the release of the secondary circuit during a predetern'iined time interval.

8. In a sound translating and amplifying systern as defined in claim 7 the addition of a rectifier circuit including an auxiliary secondary wind ing on the said input transformer, a rectifier, a parallel condenser adapted to be charged through the said rectifier, an adjustable load resistance connected across the said condenser, a

filter circuit coupled movable contact on the said load resistance, and means connecting the said movable contact with a control grid of the said electron discharge tube and adapted to impress a variable negative bias on the said control grid so as to increase the discharge current through the said electron discharge tube proportionally with the increase of the signal current.

9. In a sound translating and amplifying system, a control channel, means applying a portion of the signal energy to the said control channel, an input transformer having its secondary winding inserted in the said control channel, a pair of diode rectifiers connected in push-pull in the said control channel and adapted to rectify the control energy, a parallel condenser adapted to be charged through the said rectifier and having charge and discharge time constants of substantially the same order so as to produce a control voltage proportional to the variation of the signal energy, means impressing the said control voltage on a filter circuit comprising a blocking rectifier,

a secondary filter condenser, an electron discharge tube having its anode connected to one a parallel condenser and a coupled to terminal'fia'nd its cathode connected to the other terminal of the said filter condenser, and means impressing the said control voltage on the amplification of the said sound translating and amplifying system.

10. In a sound translating and amplifying system, a control device for variably controlling the gain of said amplifier, a primary rectifying and filtering circuit including a squar law rectifier, load resistance and having operating and releasing times of substan tially the same order, a secondary filter circuit th said primary circuit through a tapping on the said load resistance and including a filter condenser and a rectifier inserted between one terminal of th last mentioned condenser and the said tapping, a blocking circuit including a blocking condenser connected in parallel with the said load resistance of the said primary circuit, a rectifier inserted between one terminal of said blocking condenser and said load resistance,

the said secondary circuit.

11. In a sound translating and amplifying system, a control channel, means applying a portion of the signal energy to the said control channel,

an input transformer having its secondary winding inserted in the said control channel,-a pair of diode rectifiers connected in push-pull in the said control channel and adapted to rectify the control energy, a parallel condenser adapted to be charged through the said rectifier and havin charge and discharge tim constants of substantially the same order so as to produce a control voltage proportional to the variation of the si nal ener y, means impressing the said control voltage on a filter circuit comprising a blocking rectifier, a secondary filter condenser, an electron discharge tube having its anode connected to one terminal and its cathode connected to the sound translatin and amplifying system to infiuence the amplification.

KARL IIERMANN FRANZ SCHLEGEL. 

